Magnetic disks and disk drives are conventionally employed for storing data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducing heads positioned in close proximity to the recording surfaces of the disks and moved generally radially with respect thereto. Magnetic disks are usually housed in a magnetic disk unit in a stationary state with a magnetic head having a specific load elastically in contact with and pressed against the surface of the disk.
In operation, the magnetic disk is normally driven by the contact start stop (CSS) method, wherein the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates. The magnetic head unit is arranged such that the head can be freely moved in both the circumferential and radial directions of the disk in this floating state allowing data to be recorded on and retrieved from the surface of the disk at a desired position.
Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head begins to slide against the surface of the disk again and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from a stop and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. This objective becomes particularly significant as the areal recording density increases. The areal density (Mbits/in.sup.2) is the recording density per unit area and is equal to the track density (TPI) in terms of tracks per inch times (x) the linear density (BPI) in terms of bits per inch. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in closer proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head. However, another factor operates against that objective. If the head surface and recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
In order to satisfy these competing objectives, the recording surfaces of magnetic disks are conventionally provided with a roughened surface to reduce the head/disk friction by techniques referred to as "texturing." Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer which is typically chromium or a chromium-alloy, a magnetic layer, a protective overcoat which typically comprises carbon, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated on the surface of the magnetic disk.
A typical magnetic recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-base alloy, such as an aluminum-magnesium (Al--Mg) alloy, plated with a layer of amorphous nickel-phosphorous (NiP). Substrate 10 typically contains sequentially deposited thereon a chromium (Cr) underlayer 11, a magnetic layer 12 which is usually a cobalt (Co)-base alloy, a protective overcoat 13 which usually comprises carbon, and a lubricant topcoat 14. Cr underlayer 11, Co-base alloy magnetic layer 12 and protective carbon overcoat 13 are typically deposited by sputtering techniques. A conventional Al-alloy substrate is provided with a NiP plating primarily to increase the hardness of the Al substrate, serving as a suitable surface for polishing to provide the requisite surface roughness or texture, which is substantially reproduced on the disk surface.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic media in terms of coercivity, stiction squareness, low medium noise and narrow track recording performance. In addition, increasingly high density and large-capacity magnetic disks require increasingly small flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive. The requirement to further reduce the flying height of the head imposed by increasingly higher recording density and capacity render it particularly difficult to satisfy the requirements for controlled texturing to avoid head crash.
Conventional texturing techniques comprise a mechanical operation, such as polishing. See, for example, Nakamura et al., U.S. Pat. No. 5,202,810. However, conventional mechanical texturing techniques are attendant with numerous disadvantages. For example, it is extremely difficult to provide a clean textured surface due to debris formed by mechanical abrasions. Moreover, the textured surface is inevitably scratched during mechanical operations, resulting in poor glide characteristics and higher defects. In addition, various desirable substrates are difficult to process by mechanical texturing. This undesirably limiting facet of mechanical texturing, virtually excludes the use of many substrates as well as conductive graphite substrates which facilitate achieving high coercivities.
In copending application Ser. No. 08/608,072, filed on Feb. 28, 1996, a sputter texturing method is disclosed. The disclosed sputter texturing method can be advantageously applied to a plurality of different substrates.
Another alternative texturing technique to mechanical texturing comprises laser texturing by impinging a pulsed, focused laser light beam on a layer of a magnetic recording medium, such as an upper surface of a non-magnetic substrate. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, wherein the disclosed method comprises polishing an NiP plated Al substrate to a specular finish, and rotating the substrate while directing pulsed laser energy over a limited portion of the radius, thereby providing a textured landing zone leaving the data zone specular. The landing zone comprises a plurality of individual laser spots characterized by a central depression surrounded by a substantially circular raised rim.
Another laser texturing technique is reported by Baumgart et al. "A New Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995. The laser texturing technique disclosed by Baumgart et al. comprises impinging a pulsed laser light beam through a single lens focusing system on a substrate surface. Baumgart et al. disclose that the shape of the resulting protrusions is altered by adjusting the pulse energy. At low pulse energies, the bump or protrusion shape comprises a central depression and a surrounding rim, similar to that reported by Ranjan et al. As the pulse energy increases, the bottom of the depression flattens into a rounded, smooth, central dome resembling a "sombrero." At higher powers, the central dome broadens and decreases in height to eventually become equal to or lower than the rim.
A profile of a protrusion formed by the laser texturing technique as reported by Ranjan et al. is shown in FIG. 2, and comprises a substantially circular rim 23 extending above surface 21 surrounding central depression 20. The depth d of depression 20 below upper surface 21 is reported by Ranjan et al. as typically about twice the rim height h.
The variation in protrusion shape reported by Baumgart et al. is shown in FIG. 3 which depicts a sequence of atomic force microscope (AFM) cross sections of protrusions created at different incident laser pulse energies in microjoules (.mu.j).
Laser surface texturing affords an advantageous degree of control unavailable with mechanical texturing. Moreover, the accuracy of a laser light beam provides a precise delineation of the textured area boundaries, thereby enabling the accurate and reproducible formation of textured landing zones while maximizing the area available for data storage. The rounded protrusions reported by Ranjan et al. enable control of head/disk spacings while reducing friction and wear. The generally circular depressions and surrounding rims are also reported by Ranjan et al. to further reduce frictional wear by acting as areas of collection for debris and lubricant coated on the disk.
However, conventional laser texturing techniques, such as those disclosed by Ranjan et al. and Baumgart et al., suffer from several disadvantages. The geometric configuration of the topographical protrusions formed by such conventional laser texturing techniques employing a single lens focusing system result from the rapid centralized melting and thermal degradation from the center of the focused laser spot to the edge of the spot. Such single lens focusing systems generate a textured area having relatively large topographical protrusions and characterized by rather abrupt local profile changes that adversely affect the flying stability and glide performance of magnetic-recording heads, and detrimentally impact the reliability of the head-medium interface. Such problematic abrupt local profile changes require greater precision in texturing a magnetic recording medium by providing a uniform pattern of protrusions smaller than those obtained by conventional laser texturing techniques.
In copending application Ser. No. 08,647,407 filed on May 9, 1996, a method of laser texturing a magnetic recording medium is disclosed, wherein a focused laser light beam is passed through an optical crystal material interposed and spaced apart between a lens focusing system surface undergoing laser texturing. The use of an optical crystal material enables formation of a texture comprising a plurality of controlled and accurately spaced apart protrusions.
Swaminathan, U.S. Pat. No. 4,060,306 discloses the use of a multiple lens system, including an aplanatic meniscus lens and a companion doublet lens, for use in a microscope. The use of multiple lens systems for reading or writing disk drive data is disclosed by Euguchi et al., U.S. Pat. No. 5,402,407, Kurata et al., U.S. Pat. No. 5,128,914 and Endo et al., U.S. Pat. No. 5,416,755.
Accordingly, there exists a need for a texturing system, particularly a laser texturing system, capable of providing a topography comprising a plurality of controlled, relatively small protrusions of uniform height, thereby affording improved flying stability, glide performance and reliability.